The burning of carbon fuels dominates energy production and will continue to do so for the foreseeable future. Carbon sequestration is currently being pursued in an attempt to reduce CO2 emissions. However, a more elegant and long-lasting solution would be to convert emitted CO2 back into useable materials that could then be re-burned for carbon neutral energy production or used for other purposes, such as feed stock for chemical syntheses. Photosynthesis does this naturally, and programs to reduce deforestation or plant new forests and other plants have been proposed. Converting waste CO2 into usable materials at the source (such as vehicles and smokestacks) could also help. Further, there are extraterrestrial applications of CO2 conversion such as the manufacture of hydrocarbon fuels on Mars from atmospheric CO2.
Studies show that three processes, photocatalysis, electrochemistry, or photoelectrochemistry, can be used for CO2 conversion [Gattrell and Gupta, 2006]. Photocatalysis has been successfully demonstrated using photocatalysts (e.g., TiO2, ZnO and CdS). The main conversion selectivity can be challenging and depends sensitively and selectively on the properties of the photocatalysts. The products range from methane, methanol, ethylene, to formic acid and formaldehyde [Saladin et al., 1995], although the optical to chemical conversion efficiency has generally been low, less than 1 percent [Taniguchi, 1989].
Electrochemical conversion of CO2 has been based primarily on bulk metal electrodes, such as Zn, Pb, Sn, In, Cd, Cu, Au, Hg or metal alloys such as Cr—Ni—Mo. The main conversion products include formic acid, oxalic acid, methane, and hydrogen [Taniguchi, 1989]. Semiconductor electrodes TiO2 or GaAs [Monnier et al., 1980] can also generate methane and methanol, with chemical conversion levels as high as 100 percent [Canfield and Frese, 1983]. However, such an electrochemical approach requires the use of an electricity source with limited device platforms, and product selectivity is generally low.
On the other hand, photoelectrochemistry has also been demonstrated successfully in the conversion of CO2 into hydrocarbons such as methane, methanol, and ethylene, using mainly bulk p-type semiconductors as electrode materials, e.g., InP, GaAs and CdTe[Ito et al., 1984]. While the electrochemical efficiency is often high (>30 percent), the light conversion efficiency is usually low (<1 percent).
Although not heretofor applied to CO2 conversion, the idea of using solar-generated electricity to power a photoelectrochemical cell (PEC) was first proposed and demonstrated using a silicon solar cell to produce hydrogen from water in a PEC cell using a bulk TiO2 photoanode [Morisaki et al., 1976]. A more recent example using a dye-sensitized solar cell to provide power for water splitting in a PEC was described by Sivula et al. (“New nanostructures enhance solar water splitting with hematite,” SPIE Newsroom, 10.1117/2.1201007.003145,2010).